Superalloys incorporating precipitated topologically close-packed phases

ABSTRACT

The nickel-base and cobalt-base superalloys are formed to provide, in addition to the usual strengthening mechanisms, additional strengthening and other property improvements through the precipitation of copious quantities of the topologically close-packed phases, such as the sigma phase.

United States Patent [72] Inventors David H. Wagner Portland;

David N. Duhl, Newington; Edward H. Van Der Molen, East Haddam; Cornelius P. Sullivan, Branford, all 0! Conn.

Sept. 26, 1969 Nov. 16, 1971 United Aircraft Corporation East Hartford, Conn.

[2]] Appl. No. [22] Filed [45] Patented [73] Assignee [54] SUPERALLOYS INCORPORATING PRECIPITATED TOPOLOGICALLY CLOSE- PACKED PHASES 8 Claims, No Drawings [52] U.S. Cl 148/325, 75/171, l48/l2.7, 148/158, l48/l62 51 1111.01 ..C22c19/00 501 FieldofSearch 75/171; 148/127, 158, 162, 32, 32.5

[56] References Cited UNITED STATES PATENTS 3,147,155 9/1964 Lamb 75/171 Primary Examiner-Richard 0. Dean Attorney-Richard N. James ABSTRACT: The nickel-base and cobalt-base superalloys are formed to provide, in addition to the usual strengthening mechanisms, additional strengthening and other property improvements through the precipitation of copious quantities of the topologically close-packed phases, such as the sigma phase.

SUPERALLOYS INCORPORATING PRECIPITATED TOPOLOGICALLY CLOSE-PACKED PHASES BACKGROUND OF THE INVENTION The present invention relates in general to certain nickelbase and cobalt-base superalloy components, particularly those having substantial utility in gas turbine engine application.

The superalloys in general consist of those alloys exhibiting very high strengths at the temperatures associated with gas turbine engine use. They typically comprise a solid solution strengthened nickel or cobalt-rich matrix phase in which a carbide phase has been precipitated to strengthen grain boundaries. In addition, in the stronger nickel-base alloys, there is usually a precipitated phase, such as the intermetallic compound represented by the fonnula Ni (Al, Ti), usually referred to as the -y' phase. This phase may exist as a fine, uniformly distributed, solid state precipitate or as a globular, kidney-shaped structure resulting from a liquid-solid phase reaction. The former type of the y precipitate, because of its size and distribution, is generally the one most effective in the promotion of alloy strength.

To date the strength of the nickel-base superalloys has been basically a function of three distinct mechanisms: (1) solid solution strengthening; (2) carbide precipitation; and (3) 7' phase precipitation. Solid solution strengthening is imparted by the addition to the alloy of refractory elements with very high melting points, such as tungsten, molybdenum, tantalum, columbium and chromium. These strengtheners are primarily found in the matrix phase of the processed alloys. Carbide precipitation is promoted through the addition of carbidefonning elements and carbon to the alloy, and these may include those elements which also contribute to solid solution strengthening. In addition, titanium, zirconium and hafnium promote carbide formation. Precipitation of the phase is effected through the addition of those elements needed to form this phase, principally aluminum and titanium, although limited quantities of other elements are often found in this phase on chemical analysis.

In addition to the carbide and 7' phases in the various superalloys, it is also possible in some instances to precipitate additional phases. One class of such additional phases that may be precipitated is referred to as the topologically closepacked (TCP) phase. The phase known as sigma is a particularly well known precipitate of this general type and many of the commercially available superalloys are specifically formulated to avoid the formation of sigma. Usually a physical property degradation has been associated with the precipitation of sigma and, therefore, rigid control standards are frequently written into the superalloy material's specifications to prevent the selection of a formulation yielding precipitation of these undesirable phases.

The undesirability of the TCP phases results from the fact that they typically form as platelets or needles which provide the natural sites for mechanical weakness and, in addition, compete with the matrix phase for the solid solution strengthening elements. A detailed discussion of the sigma phase may be found in an article by E. 0. Hall et al., The Institute for Metals, Vol. 11 (1961) p. 61.

In a copending application of the present assignee, entitled Thermomechanical Strengthening of the Superalloys, Ser. No. 864,268, filed Sept. 26, 1969 there has been disclosed a process involving both thermal and defomiational treatments under carefully controlled conditions whereby the undesirability of the typical platelike sigma may be eliminated through alteration of its morphology. Thus, it is now advantageous to develop superalloy formulations adapted to processing by the above-mentioned techniques to provide additional strengthening or other useful property improvements through utilization of copious quantities of dispersed TCP precipitates in a substantially equiaxed morphology.

SUMMARY OF THE INVENTION The present invention relates in general to the superalloys in the nickel-base and cobalt-base alloy systems characterized by a fine dispersion of the topologically close-packed phases, such as sigma, in an equiaxed morphology, and to compositions and methods for providing such superalloys.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The ability of a given composition to precipitate the TCP phases is dependent upon a number of factors. In a particular nickel-base superalloy of the nominal composition, by weight, consisting of 15 percent chromium, 15 percent cobalt, 4.4 percent molybdenum, 3.4 percent titanium, 4.3 percent aluminum, 0.02 percent boron, 0.07 percent carbon, balance nickel, it has previously been determined that 4.66 (atomic Cr+ atomic Mo) 1.71 (atomic k Co) 0.61 (atomic residual Ni) as applied to the residual matrix phase, may be utilized to determine the threshold level of sigma phase precipitation. Thus, and despite the fact that the specified weight percentages for the various alloy constituents are otherwise very carefully controlled, control of the interaction between elements must also be considered. In those alloys formulated to prevent detrimental sigma phase precipitation during processing, the numerical value of the preceding formula is maintained below 2.32.

The TCP phases are intermediate phases of no precise composition. In general, the formation of such phases in the nickel-base superalloys is efi'ected by providing a sufficiency of those constituents particularly the transition elements including chromium, cobalt and molybdenum, which actively participate in the formation of the TCP phases. Similarly, depletion of the matrix phase in nickel will increase the propensity for TCP phase formation. This is conveniently accomplished by enriching the alloy in aluminum or titanium which causes more nickel to be tied up in the 7' phase thereby depleting the matrix phase in nickel. It has also been found that the addition of silicon to the superalloys will enhance the formation of the TCP phases. In the cobalt-base superalloy systems, tungsten, tantalum, chromium and molybdenum are particularly effective in providing the precipitated TCP phases.

In the practice of the present invention for applications involving an environment equivalent to gas turbine engine use, the nickel-base superalloys will generally contain, by weight, 10-25 percent chromium, 10-25 percent cobalt, 0-7 percent molybdenum, 1.5-7 percent titanium, 1.5-7 percent aluminum, 0.01-0.5 percent carbon and 0003-002 percent boron. The particular quantities of the alloy constituents within the above ranges, together with the other ingredients present in the alloy, are selected to promote the precipitation of copious quantities of the TCP phases.

It is not merely sufficient, however, that the allow composition be such that TCP phase precipitation is provided. The metallurgical character of the alloy must also be such as to permit processing to provide precipitation not in the usual detrimental acicular form but in an equiaxed morphology.

The preferred thermomechanical processing utilized with the nickel-base alloys involves the following steps: (1) heat treating the alloy to solution the precipitates; (2) aging the alloy to precipitate the 7' phase to a minimum of about 25 volume percent in a homogeneous distribution having an effective interparticle spacing not exceeding about 5 microns at a temperature sufficiently high to prevent substantial precipitation of the TCP phases; (3) wann working the alloy to effect an area reduction of at least 15 percent while maintaining essentially the same volume percent and distribution of the y phase established in the aging process, usually at about the aging temperature; and (4) heat treating the alloy to cause precipitation of the TCP phases in a thermally and mechanically stable array of microcrystalline imperfections.

Thus, not only must the TCP phases be precipitated in copious amounts to be effective strengtheners or hardeners, but

precipitation of these phases should preferably occur within a range of temperatures where the phase is stable without any of the TCP phases in precipitate form. Basically, this is achieved by providing a formulation wherein the 'y' solvus temperature of the alloy is above and, preferably, considerably above the precipitation temperature of the TCP phases. In any event, the precipitation mechanism must be such in the nickel-base superalloy systems that precipitation of the phase must occur, if not prior to, more expeditiously than the TCP phase precipitation since the phase morphology and distribution is the controlling factor in providing the desired TCP phase morphology after processing. In a base composition of about, by weight, 16.5 percent chromium, 16.5 percent cobalt, 5.2 percent molybdenum, 5.0 percent aluminum, 4.6 percent titanium, 0.005 percent boron, 0.025 percent carbon, balance nickel, a high y solvus temperature may be effected by providing a high aluminum plus titanium content in the alloy, typically about 9 percent. It is usually advisable to maintain the aluminum to titanium ratio at or near unity.

EXAMPLE Two bars were formulated to the following chemistry:

The alloys were vacuum induction melted and cast as electrodes which were subsequently consumable arc melted under vacuum into two l-inch-diameter bars. The bars were solution annealed at 2,150 F. for four hours and fast air-cooled. Bar A was then heattreated at l,975 F. for four hours to precipitate the 7' phase, and it was swaged to a 60 percent area reduction at 1,975", F. using a reduction of six percent per pass with a l-minute reheat between passes. Bar B was given a precipitation heat treatment at 1,875" F. but no deformation, and it was used as the standard. Bar A was given final heat treatments of 4 and 24 hours at temperatures of 1,875 F-1,200 F.

The hardness of the warm-worked material was significantly higher than that of conventional nickel base superalloys. Hardness values in the R 50s were obtained after aging at temperatures below 1,700 F. For comparison the hardness of fully heattreated Udimet 700 is about R, 39. Furthermore the yield strength of the warm-worked material in compression was significantly higher than the conventional superalloys. At room temperature the ductility of the hardest warm-worked specimen is above that of tool steel although the yield strength was 100,000 p.s.i. lower. However, at 1,000 F. where the strength of the tool steel was sharply reduced, the strength of the warm-worked alloys with the/TCP phases lost very little strength. Thus, it was clearly demonstrated that, not only may the harmful efi'ects of the platelike sigma precipitate be alleviated in the superalloys, but also significant property improvements are resultant from the use of the TCP phases as precipitated in a substantially equiaxed morphology.

Exemplary of additional compositions providing copious quantities of TCP precipitates and amenable to processing by thermomechanical means to provide the equiaxed TCP morphology are the following:

3.5%Mo, 7.0%Cb, 2.0%W, 20%Fc, 0.005%8, 0.05%C, bal. Ni.

Alloy 6 Alloy 1 Alloy s As may be seen from the foregoing examples, the nickelbase superalloy formulations particularly susceptible to strengthening and hardening improvements utilizing TCP phase precipitation generally contain about, by weight, 10-25 percent chromium for oxidation resistance, 10-25 percent 1 cobalt, O7 percent molybdenum, 1.5-7 percent titanium,

1.5-7 percent aluminum, 0.0l-0.5 percent carbon and 0.003-0.02 percent boron. To these alloys may be added various other refractory metals such as tungsten, columbium, or tantalum for strengthening and silicon to promote TCP phase precipitation. Within this basic formulation reside a number of those alloys which satisfy the basic criteria necessary for TCP phase strengthening, viz, a copious quantity of TCP phase precipitate and amenability to thermomechanical processing to provide this phase in a substantially equiaxed morphology.

In the cobalt-base superalloy systems, generally higher chromium contents are usable. Thus, cobalt-base superalloys containing 20-30 percent chromium, 5-20 percent nickel, 10-20 percent tungsten or molybdenum, 0-15 percent tantalum, 0.25-1 percent silicon, and 0.05-1 percent carbon precipitating copious quantities of the TCP phases, are suited to the present strengthening mechanism.

Particularly in the nickel-base alloy systems, the above formulations, in addition to the strengthening or hardening effect provided by the equiaxed sigma, are of fundamental interest to the gas turbine engine industry. Because TCP phase precipitation is usually associated with generally higher aluminum and ,titanium contents than conventional superalloys, low alloy densities are provided.

Although the present invention has been described in detail with reference to specific examples for the purposes of illustration, the invention in its broader aspects is not limited to the specific details described, for obvious modifications will occur to those skilled in the art.

What is claimed is:

1. An alloy article characterized by high strength and hardness at high temperatures which consists essentially of an alloy selected from the group consisting of the nickel-base and cobalt-base superalloys having uniformly dispersed therein a substantial quantity of a topologically close-packed phase in a substantially equiaxed morphology.

2. A nickel-base superalloy article characterized by high strength and hardness at high temperature which consists essentially of a nickel-base alloy matrix of high melting point, a 7' phase uniformly dispersed therein, and a dispersed topologically close-packed phase in a substantially equiaxed morphology.

3. An article according to claim 2 wherein the alloy microstructure is characterized by a polygonal substructure consisting of dislocations aligned as subcell boundaries to provide a regular array of defects.

4. An article according to claim 2 wherein the alloy microstructure comprises a randomly nonoriented dislocation distribution.

5. An article according to claim 2 wherein the allow contains, by weight, l0-25 percent chromium, 10-25 percent cobalt, 0-7 percent molybdenum, 1.5-7 percent titanium, 1.5-7 percent aluminum, 0.01-0.5 percent carbon, and 0.003-0.02 percent boron.

6. An article according to claim 5 wherein the alloy contains, by weight, 16-17 percent chromium, 16-18 percent cobalt, 5-6 percent molybdenum, 4-5 percent titanium,

4.5-5.5 percent aluminum, 0.01-0.08 percent carbon, and therein inasubstantially equiaxed morphology. 0004--01 P boron- 8. An article according to claim 7 wherein the alloy con- A cobalt base superifnoy ankle characierized y high tains, by weight, 23-27 percent chromium, 8-l2 percent strength and hardness at high temperature which consists esnickel 1347 percent tungsten molybdenum or tantalum sentially of 20-30 percent chromium, -20 percent nickel, 5 040 percem tungsten or molybdenum, 045 percent tam 0.25-0.75 percent SlllCOll, 0.050.l percent carbon and 1-2 percent manganese.

talum, 0-l percent silicon, and 0.054 percent carbon have a dispersed topologically close-packed phase precipitated 

2. A nickel-base superalloy article characterized by high strength and hardness at high temperature which consists essentially of a nickel-base alloy matrix of high melting point, a gamma '' phase uniformly dispersed therein, and a dispersed topologically close-packed phase in a substantially equiaxed morphology.
 3. An article according to claim 2 wherein the alloy microstructure is characterized by a polygonal substructure consisting of dislocations aligned as subcell boundaries to provide a regular array of defects.
 4. An article according to claim 2 wherein the alloy microstructure comprises a randomly nonoriented dislocation distribution.
 5. An article according to claim 2 wherein the allow contains, by weight, 10-25 percent chromium, 10-25 percent cobalt, 0-7 percent molybdenum, 1.5-7 percent titanium, 1.5-7 percent aluminum, 0.01-0.5 percent carbon, and 0.003-0.02 percent boron.
 6. An article according to claim 5 wherein the alloy contains, by weight, 16-17 percent chromium, 16-18 percent cobalt, 5-6 percent molybdenum, 4-5 percent titanium, 4.5-5.5 percent aluminum, 0.01-0.08 percent carbon, and 0.004-0.01 percent boron.
 7. A cobalt base superalloy article characterized by high strength and hardness at high temperature which consists essentially of 20-30 percent chromium, 5-20 percent nickel, 10-20 percent tungsten or molybdenum, 0-15 percent tantalum, 0-1 percent silicon, and 0.05-1 percent carbon have a dispersed topologically close-packed phase precipitated therein in a substantially equiaxed morphology.
 8. An article according to claim 7 wherein the alloy contains, by weight, 23-27 percent chromium, 8-12 percent nickel, 13-17 percent tungsten, molybdenum or tantalum, 0.25-0.75 percent silicon, 0.05-0.1 percent carbon and 1-2 percent manganese. 